The present invention relates to an optical disk playback system for selectively playing back information on plural types of optical disks, and more particularly, to an improvement of an optical disk discriminating apparatus for automatically discriminating the type of an optical disk mounted therein.
As is well known, not only sound data, but also moving picture data has been compressed on, for example, an optical disk whose diameter is 12 cm. This is the same diameter as a sound CD (Compact Disk). For example, a CD-ROM (Read Only Memory) and the like are used widely in the fields of education to store information.
Further, an optical disk generally called a DVD, which has the same diameter as that of a CD, has been developed, which encodes encode by compression and stores moving picture data corresponding to a movie of about two hours, sound data of eight different languages, and sub-picture data showing captions in thirty two different languages.
For this reason, it is requested now to develop an optical disk playback system of a CD/DVD convertible type which allows a CD and a DVD to be selectively mounted therein and played back on a system for playing back an optical disk. In this type of an optical disk playback system it is necessary to automatically discriminate whether a CD on a DVD is mounted therein, and switch the state of the optical system and the electric processing system to a state corresponding to the CD or the DVD.
A conventional optical disk discriminating apparatus for automatically discriminating between a CD and a DVD as described above, allows its optical pickup to execute the focus search operation to forcefully move an objective lens from its initial position, while stopping rotation of the mounted optical disk. In accordance with a focus error signal obtained during the focus search operation, and a total reflection signal obtained by photolectrically converting total reflection light from the optical disks the apparatus determines whether the mounted optical disk is a CD or a DVD.
That is, FIG. 1 shows waveforms of focus error signals and total reflection signals obtained when the focus search operation is executed for four different types of optical disks, for example, a CD, a CD-RW (ReWritable), a single-layer DVD and a double-layer DVD. Since signal waveforms associated with these types of optional disks have unique forms levels, it is possible to determine the type of the optical disk by referring to these characteristics.
A focus error signal and a total reflection signal of, for example, a CD, will be considered as reference. Differences between a CD and a single-layer DVD cannot be determined since the focus error signal level of the single-layer DVD is substantially the same as that of the CD, but slight differences can be determined since the total reflection signal level thereof is only half as high as that of the CD. Therefore, when the focus error signal level is not low but the total reflection signal level is low, it is possible to determine that the that has a high possibility of being a single-layer DVD at high possibility.
In the case of the CD-RW, both the focus error signal level and the total reflection signal level are about one third as high as those of the CD. However, since the focus error signal level and the total reflection signal level of the double-layer DVD are also comparably lower to those of the CD-RW, it is only possible to determine that the optical disk is not a CD or a single-layer DVD with reference to the only signal levels.
For this reason, discrimination between the CD-RW and the double-layer DVD needs to be executed by considering in combination the characteristic that the number of the S letter properties of the focus error signal is different (one in the CD-RW, and two in the double-layer DVD) and the characteristic that the total reflection signal level of the double-layer DVD is higher than that of the CD-RW.
When the type of the optical disk is determined in this manner, for example, the optical system of the optical pickup and the operation mode of the signal processing system are changed in response to the results of the determination, in the optical disk playback system.
In the above-described conventional optical disk discriminating apparatus, however, laser light is concentrated at one point of the optical disk surface since the focus search operation is executed in a state in which the optical disk is stopped. Therefore, in the optical disk whose data can be rewritten such as a DVD-RAM (Random Access Memory), a problem arises that writing is executed at a point where the light is concentrated, with intensity of the laser light irradiated from the optical pickup for the playback.
Further, in the conventional optical disk discriminating apparatus, the focus search operation is executed to produce a focus error signal and a total reflection signal, and the type of the optical disk is determined on the basis of the signal waveform and the level. Therefore, another problem arises in that variations in the waveform, the level of the produced focus error signal, and the total reflection signal, which are generated by scratch and contamination on the optical disk, may cause an erroneous determination.
In this case, if the tracking servo of three beams for a CD is applied to, for example, a DVD, by erroneously determining the type of the optical disk, the correct beam pitches are not applied, and only direct current offset may be unnecessarily output from the optical pickup. In this situation, large current flows to a tracking actuator coil for moving the objective lens in the tracking direction, which is very dangerous.
Moreover, some of the conventional optical disk playback systems include a measurement of protection that even if the optical system and the electric processing system are erroneously set based upon erroneously determining the type of the optical disk. When a the information from the optical disk is incorrectly read for a predetermined period, it is judged that the optical disk has been discriminated erroneously determined. As a result, optical system and the electric processing system are changed to other systems.
However, even if such protection is provided, probability of an erroneous determination of the optical disk is not lowered. Therefore, inconvenience and a waste of time occurs each time the erroneous discrimination is made.